RBP4 and type 2 diabetes mellitus: However, two other studies also reported u-shaped associations in a prediabetic study population (70.8% women)55 and in the hitherto largest prospective study (n=2091, 41.1% male).56 Cho et al57 suggested a positive association of serum RBP4 levels with T2D risk in normoglycemic and prediabetic individuals but did not investigate potential sex differences.